1. Field of the Invention
The present invention relates to gamma ray detection, and more specifically, it relates to techniques for locating a hidden or lost gamma-ray source.
2. Description of Related Art
The general problem of finding a lost or hidden radiation source is of great interest. Prior to this invention, best practices made use of large, vehicle-mounted, radiation detectors driven through a region of interest in a regular search pattern. However, fluctuations in the naturally-occurring background radiation field limit the sensitivity to which such a search can be conducted. The background radiation field varies from place-to-place by factors of 2 or more. Such fluctuations mask the signature from weak sources that would otherwise be statistically detectable. That is, the number of radiation counts seen from the source would be statistically sufficient to say that a source was present, if one only knew what the background count rate was. This problem limits source detection to those situations where the detected count rate is comparable or stronger than any normal background radiation levels. A natural corollary of this observation is the fact that for large detectors, making the detector larger does not improve the search sensitivity. By using imaging radiation detectors to conduct the search, the source shows up as a localized point which can be distinguished from the varying background.
A gamma-ray imager significantly larger than standard search instruments has demonstrated the viability of the technique of using imaging radiation detectors. At the gamma-ray energies of interest (˜50 keV to ˜5 MeV), the penetrating nature of the radiation means that no wide field-of-view, direct-imaging optics such as lenses or mirrors are known. Hence, the instrument was based on the use of the coded-aperture, indirect imaging technique. This method of imaging penetrating radiation relies on a shadow mask to project a shadow pattern onto a position-sensitive detector. Images are formed using mathematical algorithms that compare the measured shadow pattern to the known shadow mask pattern. This instrument was designed to scan on just one side of the instrument using a single shadow mask paired with a single detector plane detector.
An improvement to such an instrument makes use of two exposures where the mask pattern is inverted to its “anti-mask” (the open and closed holes in the mask are exchanged) between the equal-time exposures. This is a very effective technique to remove artifacts from the image due to changes in counts versus location seen in the detector that are not due to the mask pattern. Sources of such variation include radiation scattered off of camera components and background radiation that exposes the detectors non-uniformly. One means of processing such data is to subtract the “raw” anti-mask data from the “raw” mask data set and apply the deconvolution algorithm to the resulting differenced data set. If the order in which the subtraction is performed is such that the mask data is subtracted from the anti-mask data, then an inverse (negative) image is obtained. Such a device, although an improvement over prior techniques, is still unwieldy and time consuming, requiring a first pass along a route under test after which the mask configuration is changed to the anti-mask configuration, and then the vehicle must return to the start of the route and make a second pass thereon.
A desired device and method for simultaneously collecting mask and anti-mask gamma-ray data for determining the location of a lost or hidden gamma-ray source is desirable and such is provided by the present invention.